I use a variety of release words...my main one is okay but I think, if I could start over I would choose a different word...I have been transitioning to use the word "break" as a release.
When teaching a release word, say "break" and toss a treat to the side or in front - your dog should catch on pretty quick.
The previous poster gave you good advice!
If they are small enough, you can try using a stool, bench or table so you are working on the same level at first. Or you can just sit on the ground and work with your dog from there - until they will hold a stay for extended periods of time - then go back to standing up. I would definately have them on-leash at this point so you can keep them focused and correct them if need be!